The invention relates to a rod-core transformer according to Patent Claim 1 and to a discharge lamp cap having such a rod-core transformer, as well as to a high-pressure discharge lamp which does not contain any mercury for a motor vehicle headlight.
The International Patent Application PCT/EP00/02608 with the Publication Number WO 00/59269 discloses a gas-discharge lamp cap having a starting device which has a torroidal-core transformer arranged in the cap. The torroidal-core transformer is used for producing the starting voltage for the gas-discharge lamp.
The object of the invention is to provide a transformer which is suitable for use as a starting transformer in a lamp cap and whose resistive losses in the windings are as low as possible. In particular, the transformer is intended to be capable of use as a starting transformer for a halogen metal-vapour high-pressure discharge lamp which does not contain mercury and whose operating voltage is approximately 50% lower than that of a corresponding halogen metal-vapour high-pressure discharge lamp which does contain mercury.
According to the invention, this object is achieved by the features of Patent Claim 1. Particularly advantageous embodiments of the invention are described in the dependent patent claims.
The transformer according to the invention is in the form of a rod-core transformer and has at least one primary winding,
at least two cores which are in the form of rods, are arranged with their longitudinal axes offset parallel alongside one another, and are composed of an electrically insulating material,
a first secondary winding whose turns are arranged on the first core, with the first secondary winding having a number of parallel-connected layers of turns, and each layer of turns being arranged without any offset above the next lower layer, so that each turn in any given layer is arranged exactly above the corresponding turn of the layer located immediately underneath it of the first secondary winding,
at least one further secondary winding whose turns are arranged on at least one further core which is in the form of a rod, with the second secondary winding having a number of parallel-connected layers of turns, and each layer of turns being arranged without any offset above the next lower layer, so that each turn in any given layer is arranged exactly above the corresponding turn of the layer located immediately underneath it of the at least one further secondary winding, with
the secondary windings being electrically conductively connected to one another, and the total resistance of the secondary windings being less than or equal to 2xcexa9.
The embodiment of the transformer as a rod-core transformer allows this transformer to be inserted more easily into the lamp cap than, for example, a torroidal-core transformer, by means of an automatic placement machine. In order to minimize the power losses in the windings of the rod-core transformer and, furthermore, also to make it possible to generate a sufficiently high voltage for starting the discharge lamp, the rod-core transformer according to the invention has a number of secondary windings which are electrically conductively connected to one another and are fitted on at least two cores which are in the form of rods and are arranged with their longitudinal axes offset parallel alongside one another, with the total resistance of the secondary windings being less than or equal to 2xcexa9. The secondary windings each have a number of layers of turns which are wound one on top of the other and are connected in parallel, in order to ensure that the current flow on the secondary side is still sufficiently high even if the transformation ratio is high. In order to prevent electrical flashovers from occurring between the turns of different layers of a secondary winding, the layers of a secondary winding are arranged exactly one above the other, without any offset. There is therefore no voltage difference between the corresponding turns of the layers, which are arranged one above the other, of the secondary winding. Furthermore, the secondary windings wound in this way have only a small capacitance, so that the rod-core transformer according to the invention is also suitable for operation in the Megahertz band.
The secondary windings that are arranged on different cores which are in the form of rods are preferably connected in series. Their induction voltages are thus added to one another, and a higher starting voltage for the discharge lamp is available on the secondary side of the rod-core transformer. If the secondary windings are not all required in order to produce an induction voltage that is as high as possible, a number of secondary windings or even all the secondary windings can be connected in parallel, in order in this way to reduce the total resistance of the secondary windings.
The cores, which are in the form of rods, of the transformer according to the invention are advantageously in the form of ferrites and, in particular, as nickel-zinc sintered ferrites, owing to their high relative permeability. Ferrites such as these are composed of a sintered nickel-zinc mixed oxide, which has a comparatively high electrical resistivity of approximately 105 xcexa9m. The ferrites may thus, de facto, be regarded as electrical insulators. They ensure that the rod-core transformer has a high breakdown resistance, and thus allow the production of very high induction voltages.
The rod-core transformer according to the invention advantageously has two, and only two, cores which are in the form of rods and each have a secondary winding arranged on them, with each of these secondary windings having 50 to 200 turns. A rod-core transformer such as this has a physically compact design and has a sufficient number of turns on the secondary side to ensure a sufficiently high transformation ratio for using it as a starting transformer. The wire diameter of the secondary windings is advantageously greater than or equal to 0.1 mm, and is preferably even greater than 0.2 mm, in order to keep the total resistance of the secondary windings as low as possible. It has been found that a rod-core transformer having only two cores which are in the form of rods and having two secondary windings which each have 50 to 200 turns and whose wire diameter is greater than 0.1 mm and preferably even greater than 0.2 mm makes it possible to produce a starting transformer for producing the starting voltage for a high-pressure discharge lamp, which starting transformer has a small physical extent, so that it can be inserted into the cap of a high-pressure discharge lamp for a motor vehicle headlight, and its secondary side has a sufficiently low internal resistance in order to allow the rod-core transformer to be used to operate even high-pressure discharge lamps with a comparatively low operating voltage, such as halogen metal-vapour high-pressure discharge lamps which do not contain mercury. The secondary of the rod-core transformer has a resistance which is sufficiently low that only minor losses occur here, even when the lamp current flows through the secondary windingsxe2x80x94as is normal when used in pulsed starting devicesxe2x80x94after successfully starting the gas discharge in the lamp.
The at least one primary winding of the rod-core transformer is advantageously arranged such that the magnetic flux in two cores which are in the form of rods and are arranged alongside one another in each case runs in opposite directions. This can be achieved in a simple manner by arranging one primary winding on each core, which is in the form of a rod and is preferably composed of a ferrite, with the primary windings being connected in parallel. These measures reduce the stray field from the transformer. In addition, ferrite platelets are advantageously arranged in the ends of the ferrite cores (which are in the form of rods) and each interact with two adjacent ferrite cores (which are in the form of rods) in order to constrain the stray field of the transformer. This further reduces the losses in the rod-core transformer. The distance between the ferrite platelets and the ends of the ferrite cores, which are in the form of rods, is advantageously variable or adjustable, in order to make it possible to set the inductance of the transformer according to the invention to the desired value.
The primary winding or the primary windings of the rod-core transformer according to the invention advantageously has or have only one to three turns, in order to achieve a transformation ratio which is as high as possible and in this way to allow a correspondingly high induction voltage.
The cores, which are in the form of rods, and the secondary windings, which are arranged on them, of the rod-core transformer according to the invention are advantageously each accommodated in a separate housing, and the housings can be connected to one another by means of a plug connection. In consequence, the individual cores, which are in the form of rods, can be arranged alongside one another with well-defined spacing, and can be protected from external influences, in a simple manner. These housings are preferably composed of an electrically insulating material, such as plastic, in order to ensure that the transformer can withstand sufficiently high voltages. For the same reason, the cavities in the housing are advantageously filled with an electrically insulating encapsulation compound. The encapsulation compound preferably contains a ferrite powder which is homogeneously mixed with it, in order to improve the inductance of the rod-core transformer. The ferrite powder in the encapsulation compound may be used in addition to the ferrite platelets mentioned above, or instead of these ferrite platelets. However, it is also possible to dispense with both the ferrite platelets and the ferrite powder in the encapsulation compound.